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Extraction and mechanism for the separation of neutral N-compounds from coal tar by ionic liquids
Fuel 194 (2017) 27–35
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Full Length Article
Extraction and mechanism for the separation of neutral N-compounds from coal tar by ionic liquids Lianzheng Zhang, Dongmei Xu, Jun Gao ⇑, Shixue Zhou, Liwen Zhao, Zhishan Zhang College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The selected ILs shows good
extraction performance for neutral Ncompounds. The mechanism of the extraction was confirmed to be the formation of hydrogen bond. BmimCl can be easily regenerated and recycled with good performance even 5 times. A green approach was provided for the separation of neutral Ncompounds from coal tar.
a r t i c l e
i n f o
Article history: Received 11 October 2016 Received in revised form 28 December 2016 Accepted 29 December 2016
Keywords: Coal tar Indole Carbazole Imidazole Extraction Molecular simulation
a b s t r a c t Coal tar is one of the valuable chemical materials and energy, from which the nitrogen-containing compounds (N-compounds), indole, carbazole, pyridine, and quinoline, are mainly separated. In the present work, the imidazolium-based ILs with different anions, 1-butyl-3-methyl-imidazolium chloride (BmimCl), 1-butyl-3-methyl-imidazolium bromide (BmimBr), 1-butyl-3-methyl-imidazolium tetrafluoroborate (BmimBF4), 1-butyl-3-methyl-imidazolium disulfate (BmimHSO4), and 1-butyl-3-methylimidazolium acetate (BmimCH3COO), were used to separate those N-compounds via extraction. The performances of ILs extraction were evaluated by the extraction efficiency and distribution coefficient. High extraction efficiency and selectivity could be found for the neutral N-compounds, indole and carbazole, by BmimCl than basic N-compounds, pyridine and quinoline. Meanwhile, the optimized conditions were ascertained and the selected ILs could be easily regenerated by water and be sustainable recycled by a back-extraction process. Furthermore, a hydrogen bond was formed between the neutral N-compound and the selected ILs, and the mechanism was confirmed based on the analysis by a molecular simulation. Thus, an approach was provided for the separation of neutral N-compounds from coal tar. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Coal tar is one of the valuable chemical materials and energy with many compounds, which have already been identified more than 500 compounds. Currently, coal tar is generated from the pro⇑ Corresponding author. E-mail address: [email protected] (J. Gao). http://dx.doi.org/10.1016/j.fuel.2016.12.095 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
cess of coal pyrolysis, which can be classified into three categories according to the pyrolysis temperature, they are the lowtemperature coal tar (LTCT), the middle-temperature coal tar (MTCT), and the high-temperature coal tar (HTCT). The compositions of LTCT and MTCT are distinctly different with HTCT. And the compounds from coal tar have extensive applications in various fields [1]. As a result, the chemicals from coal tar attracts more and more attention. Especially in China, coal-derived chemicals
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play a significant role in the chemical industry. And many fine chemical reagents are mainly separated from coal tar, also coal tar could be used in the preparation of fuel oil by hydrogenation [2–6]. According to the reference data [7,8], the production of LTCT and MTCT were about 2 million tons in 2008, and increased to 8.61 million tons in 2012 in China. Thus, considering the huge production of coal tar, the development of separation method for the different kinds of compounds with relative high content from coal tar is imperative. Coal tar is a complex mixture that includes many kinds of heteroatom compounds which are different types phenol compounds [9,10], basic nitrogen compounds [11–14], neutral nitrogen compounds [7,15], neutral aliphatic and aromatic compounds [16– 19], etc. The concrete composition of coal tar varies with different coal types and coal pyrolysis technologies. In chemical industry, the phenol compounds mainly derived from coal tar and have extensive applications [20–22]. Commonly, the separation of phenol compounds from coal tar can be achieved by traditional methods, in which some chemical reactions were performed with strong alkaline and acidic aqueous solutions [23,24]. Similarly, coal tar is also the main source of N-compounds, such as indole, carbazole, pyridine and quinoline (see Fig. 1), in the chemical industry, which have been extensively applied in the productions of medicine, pesticides, spice, dyestuff, and plastics. Particularly, due to the chemical properties of indole, numerous chemical reactions can be achieved. Such as electrophilic aromatic substitution reaction, oxidation reaction, or applied in generating derivatives for further organic synthesis [25]. There are many non-catalytic processes that have been adopted for the separation of N-compounds from coal tar, such as alkali fusion (potassium hydroxide) [26], sulfuric acid washing, solvent extraction [1,27], ion-exchange resin separation [28,29], and liquid-liquid extraction by volatile carboxylic acid [30]. Though the above methods are widely accepted, there still exist some drawbacks or limits because some processes discharge a large amount of waste water, the equipment are corroded, or the selectivity is lower. Thus, for separation of the N-compounds, more efficient and environmental friendly methods, which have lower cost and higher selectivity than the common adopted methods are needed. Generally, liquid-liquid extraction has been adopted in many separation fields. But for the separation of fine chemicals from coal tar, some drawbacks limit the application of this method, since the organic solvents used are volatile, and the selectivity and efficiency are low. Compared to the common organic solvents, ionic liquids (ILs) are boomingly investigated and applied in variety fields [31]. Due to its nonvolatile, good thermosability, strong dissolving capacity, designability, selectivity, and environment friendly, the applications of ILs in green chemical processes were investigated by many researchers [20,22,32–37]. To our knowledge, few investigations were done for the separation of N-compounds form coal tar. Jiao et al. [38] used several imidazolium-based ILs as extraction
agents to separate indole from wash oil. Habaki et al. [39] discussed the separation for the coal tar absorption oil by an ionic liquid supported liquid membrane. And Su et al. [40] used two series phosphate-based alkylimidazolium ILs, dialkyl and dihydrogen phosphate alkylimidazolium ILs to separate nitrogen compounds from the coal tar diesel fraction by extraction. All of them showed good results. Moreover, many studies for denitrogenation from diesel and gasoline by different ILs indicated the feasibility of liquid-liquid extraction by ILs [41–45]. In this work, the imidazolium-based ILs with different anions, Cl , Br , BF 4 , HSO4 , and CH3 COO were applied to extract neutral N-compounds, indole and carbazole, from the model coal tar oil, which were immiscible with the model oil. The aim is to select one or more possible ILs as solvents with higher efficiency and lower cost for the separation of the neutral N-compounds, indole and carbazole, from coal tar. Extraction efficiency (EE) and distribution coefficient (D) were adopted to evaluate the extraction ability of the investigated ILs. Then, the main conditions that could affect the extraction process were systematically studied, such as the extraction temperature, mass ratio of the extraction agent to model oil, extraction time, and initial N-content. The optimized experimental conditions and extraction mechanism were explored. The extraction mechanism was confirmed to be the formation of a hydrogen bond between ILs and neutral N-compounds. Meanwhile, the influence of the regeneration and the recycling times of the used ILs were also studied. 2. Experimental 2.1. Materials and preparation of the model oil All chemicals used in this work were obtained commercially, which were analytical pure reagents. The purities were all refer to mass fraction which were reported by the suppliers. All of the ILs were dried by a rotary evaporator at 150 °C in vacuum conditions (Shanghai Shensheng Tech. Co., Ltd, R205D). The other reagents were all used without purification. And the relevant detailed information of the reagents is listed in Table 1. Since coal tar consists of various kinds of components, which can be broadly divided into acidic, alkaline, aromatic, and neutral compounds, respectively. Thus, indole, carbazole, pyridine, quinoline naphthalene, and acenaphthene were selected to prepare the model coal tar oil. And methylbenzene was selected as the solvent and used to prepare the solution. All the selected components were weighed accurately and then added into a beaker. Therefore, two model oils were prepared which the mass ratio were 1:1:1:10 for indole, pyridine, quinoline, and naphthalene (model oil I), and 1:1:10:5 for carbazole, pyridine, naphthalene, and acenaphthene (model oil II), respectively. For the second model coal tar oil, acetone was added to the model oil to avoid the phase splitting due to the dissolving capacity of acenaphthene and carbazole. The concentrations of the two model coal tar oils are listed in Table 2. 2.2. Equipment and extraction procedure
Fig. 1. Chemical structures of four N-compounds from coal tar.
The liquid-liquid extraction experiments were carried out by using a customized jacketed glass vessel, which had a volume of 50 ml and was connected to thermostatted water that could keep a constant temperature. A suitable amount of model oil and IL were mixed in the glass vessel, which was equipped with magnetic stirrer, and kept at a constant temperature within ±0.1 K (SYC-15Bs, produced by Nanjing Huchuan Electronic Equipment Co., Ltd.). During the experiment, the magnetic stirrer was set at 600 rpm at certain temperature for 30 min, which the temperature was set according to the different ILs, for instance, the temperature
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L. Zhang et al. / Fuel 194 (2017) 27–35 Table 1 Suppliers and mass fractions of the chemical reagents. Component
CAS
Suppliers
Purity
Purification method
Methylbenzene Pyridine Quinoline Indole Carbazole Naphthalene Acenaphthene Acetone BmimCl BmimBr BmimBF4 BmimHSO4 BmimCH3COO
108-88-3 110-86-1 91-22-5 120-72-9 86-74-8 91-20-3 83-32-9 67-64-1 79917-90-1 85100-77-2 174501-65-6 262297-13-2 284049-75-8
Tianjin Kemiou Chemical Reagent Co., Ltd. Tianjin Fuyu Fine Chemical Co., Ltd. Tianjin Bodi Chemical Co., Ltd. Shanghai Zhanyun Chemical Co., Ltd. Shandong Xiya Chemical Industry Co Ltd. Chengdu Kelong Chemical Co., Ltd. Beijing Blingwei Technology Co., Ltd. Tianjin Kemiou Chemical Reagent Co., Ltd. Linzhoukeneng material Co., Ltd Linzhoukeneng material Co., Ltd Linzhoukeneng material Co., Ltd Linzhoukeneng material Co., Ltd Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences
P99.5% P99.5% P99.0% 99.0% 99.0% P99.0% 98.0% P99.5% 99.0% 99.0% 99.0% 98.0% 99.0%
None None None None None None None None Dried Dried Dried Dried Dried
sented the extraction performance of indole and carbazole by ILs. Both of the two equations are expressed as follows:
Table 2 Concentrations of two model coal tar oils. Components
Model oil I
Model oil II
Indole Pyridine Quinoline Naphthalene Carbazole Acenaphthene
58.19 g L1 39.43 g L1 64.59 g L1 192.37 g L1 – –
– 40.34 g L1 – 401.03 g L1 39.90 g L1 160.74 g L1
range was 40–100 °C to extract indole from the model oil by BmimCl. Then, the stirring was stopped and the mixture was settled for 1 h at the same temperature. During the settlement, any disturbance from the outside should be avoided until the phase equilibrium state was reached. After that, the two layers were separated carefully and the mass of both layers were all weighted accurately. The samples of the upper phase were drawn by syringes which were analyzed accurately by GC. The procedures were similar to Hansmeier’s work [41]. In addition, the back-extraction of the lower phase were adopted by water. 2.3. Analysis All samples of the upper phase were analyzed by a gas chromatography (Lunan GC SP-6890) which was equipped with a flame ionization detector (FID) and a capillary column (DB-WAX, 30 m 0.53 mm 1.00 lm, Agilent Technologies). The analysis conditions were described as follows: the temperature of the injector, 280 °C, the temperature of the FID detector, 270 °C and the detector rang for FID, 100. Considering the large boiling-point difference for different components, a temperature programming procedure was selected here. For the model oil I, the initial temperature of oven was set at 80 °C for 2 min, then the temperature increased at a rate of 30 °C min1 to reach 200 °C, and kept for 3 min; for the model oil II, the initial temperature of oven was set at 80 °C for 2 min, then the temperature increased at a rate of 30 °C min1 to reach 220 °C, and kept for 15 min. With the N2000 workstation developed by Zhejiang University, the concentration of all samples were determined, which was calibrated by four different known mixtures gravimetrically weighed by an analytical balance with a standard uncertainty of ±0.0001 g. For the purpose of lower deviation, each sample was analyzed by GC at least three times and the mean values were adopted. 2.4. Calculation of extraction efficiency and distribution coefficient The extraction efficiency (EE) were calculated by Eq. (1), and the distribution coefficient (D) were calculated by Eq. (2), which pre-
EENcompounds ¼ ½ðC 0 C f Þ=C 0 100
ð1Þ
DNcompounds ¼ C IL =C f
ð2Þ
where in Eqs. (1) and (2), C 0 is the concentration of the original model oil and C f is concentration of the model oil after extraction. In Eq. (2) C IL is the concentration of the model oil in IL phase. And the concentration of the lower layer can be obtained by the conservation of mass. 3. Results and discussion With different combination of the cations and anions, there are vast amount of ILs that can be developed theoretically and may have extensive applications in different fields. Several researchers have tried different types of ionic liquids for the separation or removal of N-compounds from coal tar or diesel fuels [38–40,42,43,45]. Thereinto, the performance of those ILs used in the extraction process for the neutral N-compounds, which were evaluated by one extraction step, are compared and summarized in Table 3. Although there have already some investigations by some researchers, the applications of the ILs are still limited by the high cost and some related performance, only little kinds of ILs have been applied in the industry, such as the imidazolium, pyri dinium cations with halide, BF 4 or PF6 anions. So it is important to select some appropriate kinds of ILs, which could have high efficiency and low cost, to extract neutral N-compounds from coal tar. 3.1. Selection of ILs For the studied model coal tar oils, it was found that the imidazole-based ILs had the ability to extract the neutral N compounds. The ILs with BF 4 or PF6 anions could be decomposed, which corrosive HF can be generated [46–48]. Thus, alternative anions, anions, such as Cl , Br and two acidic HSO 4 , CH3 COO were studied here. Meanwhile, ILs with BF4 was also checked for comparison. The ILs with same cation, 1-butyl-3-methylimidazolium, and different anions were investigated to extract neutral N-compounds from the two model coal tar oils. The structures of the five imidazole-based ILs are shown in Fig. 2, which are 1-butyl-3-methyl-imidazolium chloride (BmimCl), 1-butyl-3methyl-imidazolium bromide (BmimBr), 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4), 1-butyl-3-methylimidazolium disulfate (BmimHSO4), and 1-butyl-3-methylimidazolium acetate (BmimCH3COO), respectively. To select the suitable ILs, the experiments were performed with mass ratio of 1:1 for different ILs to the model oil in the glass vessel
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Table 3 Comparison of some ILs for the extraction of N-compounds. ILs
N-compounds
T/K
IL:oil
Extraction time/min
Extraction efficiency/%
Ref.
BmimBF4 BmimPF6 BmimH2PO4 BmimN(CN)2 C2mimCl (CH3CH2)3N(CH2)3SO3HHSO4
Indole Indole Coal tar diesel fraction Carbazole Indole Indole
303.15 303.15 313.15 298.15 333.15 298.15
1:1 1:1 1:5 1:1 1:1 1:5
60 60 30 20 >30 90
91.36 86.47 92.3 100 76 99.1
[38] [38] [40] [42] [43] [45]
Fig. 2. Chemical structures of selected ionic liquids: (1) 1-butyl-3-methyl-imidazolium chloride (BmimCl); (2) 1-butyl-3-methyl-imidazolium bromide (BmimBr); (3) 1-butyl-3-methyl-imidazolium tetrafluoroborate (BmimBF4); (4) 1-butyl-3methyl-imidazolium disulfate (BmimHSO4); (5) 1-butyl-3-methyl-imidazolium acetate (BmimCH3COO).
at 60 °C for 1 h with vigorous stirring. The calculated extraction efficiency and distribution coefficient for the N-compounds are given in Fig. 3. It was shown that, in Fig. 3, the different anions of the ILs presented different extraction effects, which the EE and D values for the neutral N-compounds followed the order of anions: BmimCl > BmimBr > BmimHSO4 > BmimBF4 > BmimCH3COO. Moreover, the EE by BmimCl and BmimBr were 97.50, 94.70 for indole and 95.01, 85.15 for carbazole, the D values were 38.98, 17.89 for indole and 19.10, 5.76 for carbazole. The EE and D values for pyridine and quinolone were obtained to make a comparison, which the EE values were 13.24, 8.25 for pyridine and 26.40, 11.45 for quinoline, the D values were 0.15, 0.09 for pyridine and
0.36, 0.13 for quinoline by BmimCl and BmimBr in the above conditions. From the above results, good selectivity of BmimCl for indole and carbazole could be found. Thus, BmimCl was more suitable to extract neutral N-compounds from the model oils. On the contrary, all the selected ILs presented little or none extraction ability for naphthalene and acenaphthene. In addition, BmimHSO4 had a relatively higher extraction ability than BmimCl for pyridine and quinoline, which the EE could reach 83.46 for pyridine and 59.99 for quinoline. But for indole and carbazole, the extraction ability of BmimHSO4 was smaller than that of BmimCl, which the EE were 89.24 for indole and 82.13 for carbazole. Hence, based on the overall extraction efficiency for N-compounds from coal tar, the optimized extraction process utilizing BminCl for the neutral N-compounds from the model coal tar oils were explored. Due to the high extraction efficiency and distribution coefficients of BmimCl for indole and carbazole, a promising way was presented to separate the neutral N-compounds from the model coal tar oils. Hence, the different conditions, such as the extraction temperature, mass ratio of IL to model oil, extraction time, and initial N-content, that may have an influence on the extraction process by BmimCl were optimized in detail in the following part. 3.2. Optimization of extraction conditions 3.2.1. Effect of extraction temperature The influence of extraction temperature was investigated in detail by the single factor experiments. Five temperature points were designated, which were 40 °C, 50 °C, 60 °C, 80 °C, 100 °C,
Fig. 3. The extraction efficiency and distribution coefficients of different N-compounds by five ionic liquids.
L. Zhang et al. / Fuel 194 (2017) 27–35
respectively. The extraction was conducted with the mass ratio of 1:5 for 1 h. The EE and D values are presented in Fig. 4a, which indicated that the EE of indole increased slightly with the increasing of the temperature from 50 °C to 80 °C, then decreased over 80 °C. This could be caused by the solubility of indole in the methylbenzene increased when the temperature was larger than 80 °C. For carbazole, EE was insensitive to the temperature from 40 °C to 100 °C. And the D values of indole and carbazole presented the same trends as EE. For indole and carbazole, the maximum EE and D values were 89.64, 81.86 and 44.17, 21.58 at 80 °C, which only a little higher than those at 60 °C. Thus, in consideration of the industrial application, the optimum temperature was confirmed as 60 °C. And the extraction temperature was set at 60 °C for the following experiments.
3.2.2. Effect of mass ratio of IL to model oil The mass ratio of the IL to the model oil had a significantly influence on the extraction process. Therefore, the effect of mass ratio was investigated in detail and the mass ration of IL to model oil 1:10, 1:5, 1:2, 1:1, 2:1 (w:w) were selected. The extraction experiment was carried out at 60 °C for 1 h. The EE and D results indicated, as shown in Fig. 4b, that the EE values of indole and carbazole had the same trend, which increased with increasing the mass ratio, and EE increased fast from the mass ratio 1:10 to 1:2. Eventually, the EE values of indole and carbazole reached to 98.32 and 96.65, respectively at the mass ratio of 2:1. On the contrary, D for indole and carbazole had different trend with each other, which D for indole increased with increasing the mass ration and reached the maximum value 42.20 at the mass ratio of 1:2, but for carbazole, it decreased with the increase of the mass ratio from 1:10 to 2:1. Thus, considering the extraction effect and the cost of the ionic liquid, the mass ratio of 1:5 was selected in the following experiments.
3.2.3. Effect of extraction time The influence of extraction time was investigated in detail from 5 to 90 min, which 5, 10, 30, 60, 90 min were selected. All the extraction experiments were conducted at 60 °C with the mass ratio of 1:5. The results are presented in Fig. 4c, which the EE and D values for both indole and carbazole increased with increasing the extraction time. The EE and D values increased rapidly and reached to 90.10, 47.09 for indole and 83.14, 27.66 for carbazole, respectively, as the extraction time increased to 30 min. Then, the EE and D curves flattened as the extraction time increased to 90 min. According to the above experiment results, at least 10 min was needed for the extraction to reach the higher extraction efficiency. Therefore, 30 min was adopted to assure the extraction in this study.
3.2.4. Effect of initial neutral N-content Considering the different contents of neutral N-compounds in the different coal tar, several model oils with different initial content for the neutral N-compounds were prepared to explore the influence of the initial neutral N-content for the extraction by BmimCl, which the initial neutral N-content varied from 2% to 16%. Moreover, the extraction experiments were performed under the mass ratio of 1:5 (w/w) for ILs to model oil at 60 °C for 30 min. As shown in Fig. 4d, both EE and D had the same trend which decreased slightly with the increasing the initial neutral Ncontent. The extraction efficiency varied from 90.99% to 86.83% for indole and 83.43% to 76.47% for carbazole with the change of the N-content from 2% to 10%, which indicates the different initial contents of indole and carbazole in different model oils have less influence on the extraction process.
31
3.3. Mechanism of the extraction process Based on the above experimental results, the interactions between the ILs and the neutral N-compounds accounted for the high extraction efficiency and selectivity of indole and carbazole. Many researchers have reported that the interactions existed between the ILs and aromatic hydrocarbons are p-p interactions by a so-called sandwich-like structure [32,41,49], the clathrate formed between the cations of the ILs and the enclosed solvent [50], or hydrogen bonds caused by the anions of the ILs [51,52], which may give an explanation for the high extraction ability of the investigated ILs. In addition, Wang et al. [47], regarded the extraction mechanism was not only liquid-liquid extraction, but also the acidity of the ILs. In this study, the same cation, 1-butyl-3-methyl-imidazolium, was adopted to reduce the other interaction influences. And different anions were adopted to investigate the extraction of neutral Ncompounds from the two model coal tar oils. The investigated ILs, BmimCl and BmimHSO4, had a higher extraction ability to extract indole and carbazole from the model oils, which could be explained by the intermolecular forces between the ILs and the neutral Ncompounds. Hetero-atoms, in indole and carbazole, had the possibility to form hydrogen bond with the cations or anions of different ILs. To verify the hypothesis, the chemical bonds formed between the ILs and the neutral N-compounds were simulated by using the DMol3 module incorporated the density functional theory (DFT) [53] which is available in Accelrys’ Materials studio. First, the geometry optimization of the neutral N-compounds, ILs and their complexus were done by the GGA/VWN-BP function with the DNP 4.4 basis set. To achieve rapid convergence, the smearing energy was set at 0.005 Ha. Meanwhile, the convergence criteria self-consistent field (SCF) tolerance was set at 1.0 ⁄ 106 Ha, and the convergence tolerance were set as follows: energy tolerance of 1.0 ⁄ 105, maximum force of 0.002 Ha/Å, maximum displacement of 0.005 Å. The dielectric constant of the solvent, methylbenzene, was 2.4. The parameter ‘‘Charge” were set as 1 or 1 for the cations and anions of different ionic liquids, respectively. As shown in Fig. 5, the bond lengths are 2.119 Å between BmimCl and indole (a), 2.126 Å between BmimCl and carbazole (b), respectively, which indicated that a hydrogen bond, NAH Cl, was formed between the anion and indole/carbazole. Meanwhile, in order to confirm the hydrogen bond between the anion of the IL and the neutral N-compounds, the electron densities are shown in Figs. 6 and 7, which reflected the formation of the hydrogen bond. The total densities, as shown in Fig. 6, indicate that the NAH Cl hydrogen bonds between indole/carbazole and anion are still evident, even if the isovalue is 0.2. Also, the similar result was obtained by the deformation density as shown in Fig. 7. In Fig. 7, a certain degree of H Cl bonding interactions is observed from the slice of the deformation density, which the received electron area encircling around Cl atom is expressed in red and the betatopic area encircling around H atom is expressed in blue for indole and carbazole with the anion. Similarly, a hydrogen bond was formed between indole/carbazole and BmimHSO4 with the bond lengths of 1.894 Å, 1.899 Å, respectively. And the hydrogen bonds of NAH O between indole/carbazole and anion are evident even if the isovalue is 0.15, which the corresponding bond lengths and electron density maps are both presented in the Supplementary Material (Fig. A. 1–3). On the contrary, pyridine and quinoline, which belongs to basic N-compounds, could not form the hydrogen bond with the anions due to the lack of hydrogen atom connected to the nitrogen atom. Therefore, based on the MS simulation results, the formation of the hydrogen bond between the anions and the hetero-atom presents the mechanism of the extraction process.
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Fig. 4. EE and D values vs. temperature (a), mass ratio of IL to model oil (b), extraction time (c), and initial N-content (d) for the extraction of indole (j) and carbazole (N).
Fig. 5. Bond lengths between BmimCl and indole (a), and between BmimCl and carbazole (b).
L. Zhang et al. / Fuel 194 (2017) 27–35
33
Fig. 6. Total density maps of different isovalues for BmimCl with Indole (a, b, c) and Carbazole (d, e, f).
Fig. 7. Deformation charge density maps for BmimCl with Indole (a) and Carbazole (b).
3.4. Regeneration and recycling ionic liquid Considering the application of the extraction of indole and carbazole from coal tar oil by ILs in the industry, it is of importance to regenerate and reuse the ILs when the extraction process is applied. Eßer et al. reported that the neutral N-compounds could be separated from the extract by distillation [54]. In view of the high boiling temperatures of indole and carbazole, a backextraction process for indole and carbazole was adopted here. For the back-extraction process, water was selected as the backextraction agent. When the two phases were formed after the extraction, the upper was the oil phase and the lower was IL phase with the extracted neutral N-compounds. The two phases were separated by a separating funnel. Then, a certain amount of water was added into the IL phase with continuous stirring. After settling for a certain time, the neutral N-compounds precipitated and were
filtrated. The ILs was regenerated by a rotary evaporator at 150 °C in vacuum conditions, until the mass of the ILs was constant. The recycled BmimCl was reused to extract the neutral Ncompounds from the model oils. The extraction efficiencies of neutral N-compounds were not lowered even recycled 5 times, which is shown in Fig. 8. The IL could be easily regenerated and be sustainable recycled with good extraction performance. Meanwhile, the regenerated IL was checked by FT-IR, which is shown in Fig. 9. The spectra of the recycled IL were compared with the new IL, which the absorption peaks had no difference.
4. Conclusion In this work, an approach was provided for the separation of neutral N-compounds, indole and carbazole, from coal tar by ionic
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The extraction efficiency of BmimCl was not changed obviously and can be sustainable recycled with good extraction performance even it was recycled five times.
Acknowledgement The authors are grateful to the support of National Natural Science Foundation of China [Grant No. 21306106] and Shenzhen Supercomputer Center (DMol3 and Reflex modules of Materials Studio 7.0).
Appendix A. Supplementary material
Fig. 8. Extraction efficiency vs. recycle times of BmimCl for indole and carbazole model oils (temperature, 60 °C; mass ratio of IL to oil, 1:1; time, 1 h).
The bond lengths and electron density maps of the hydrogen bonds of NAH O between indole/carbazole and BmimHSO4 are both presented in the supplementary material. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.12.095.
References
Fig. 9. The FT-IR spectroscopy of the original IL and recycled IL.
liquids. The extraction effect of five imidazolium-based ionic liquids with different anions, BmimCl, BmimBr, BmimBF4, BmimHSO4, and BmimCH3COO, were explored. All of those five ILs had the extraction ability for the neutral N-compounds. The extraction efficiency and distribution coefficient were adopted to evaluate the extraction ability, which the extraction performance follows the order of anions: Cl > Br > HSO 4 > BF4 CH3 COO . And the high extraction ability and selectivity was confirmed for the neutral N-compounds by BmimCl from the model coal tar oil, which the distribution coefficient were 38.98 and 19.10 for indole and carbazole, 0.15, 0.36 for pyridine and quinoline, respectively. Furthermore, the conditions of the extraction were investigated in detail. The extraction efficiency could reach 98.32 and 96.65 for indole and carbazole at 60 °C with mass ratio of BmimCl to model coal tar oil, 2:1, and extraction time, 1 h. And the EE and D values decreased slightly with increasing of the initial N-content. The mechanism of the extraction of indole and carbazole from coal tar oil by ILs was confirmed to be the formation of the hydrogen bonds between ILs and neutral N-compounds, which were simulated by using the DMol3 module. Meanwhile, the regeneration and recycle of the BmimCl were investigated and water was selected as the back-extraction agent.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Extraction and mechanism for the separation of neutral N-compounds from coal tar by ionic liquids Lianzheng Zhang, Dongmei Xu, Jun Gao ⇑, Shixue Zhou, Liwen Zhao, Zhishan Zhang College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The selected ILs shows good
extraction performance for neutral Ncompounds. The mechanism of the extraction was confirmed to be the formation of hydrogen bond. BmimCl can be easily regenerated and recycled with good performance even 5 times. A green approach was provided for the separation of neutral Ncompounds from coal tar.
a r t i c l e
i n f o
Article history: Received 11 October 2016 Received in revised form 28 December 2016 Accepted 29 December 2016
Keywords: Coal tar Indole Carbazole Imidazole Extraction Molecular simulation
a b s t r a c t Coal tar is one of the valuable chemical materials and energy, from which the nitrogen-containing compounds (N-compounds), indole, carbazole, pyridine, and quinoline, are mainly separated. In the present work, the imidazolium-based ILs with different anions, 1-butyl-3-methyl-imidazolium chloride (BmimCl), 1-butyl-3-methyl-imidazolium bromide (BmimBr), 1-butyl-3-methyl-imidazolium tetrafluoroborate (BmimBF4), 1-butyl-3-methyl-imidazolium disulfate (BmimHSO4), and 1-butyl-3-methylimidazolium acetate (BmimCH3COO), were used to separate those N-compounds via extraction. The performances of ILs extraction were evaluated by the extraction efficiency and distribution coefficient. High extraction efficiency and selectivity could be found for the neutral N-compounds, indole and carbazole, by BmimCl than basic N-compounds, pyridine and quinoline. Meanwhile, the optimized conditions were ascertained and the selected ILs could be easily regenerated by water and be sustainable recycled by a back-extraction process. Furthermore, a hydrogen bond was formed between the neutral N-compound and the selected ILs, and the mechanism was confirmed based on the analysis by a molecular simulation. Thus, an approach was provided for the separation of neutral N-compounds from coal tar. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Coal tar is one of the valuable chemical materials and energy with many compounds, which have already been identified more than 500 compounds. Currently, coal tar is generated from the pro⇑ Corresponding author. E-mail address: [email protected] (J. Gao). http://dx.doi.org/10.1016/j.fuel.2016.12.095 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
cess of coal pyrolysis, which can be classified into three categories according to the pyrolysis temperature, they are the lowtemperature coal tar (LTCT), the middle-temperature coal tar (MTCT), and the high-temperature coal tar (HTCT). The compositions of LTCT and MTCT are distinctly different with HTCT. And the compounds from coal tar have extensive applications in various fields [1]. As a result, the chemicals from coal tar attracts more and more attention. Especially in China, coal-derived chemicals
28
L. Zhang et al. / Fuel 194 (2017) 27–35
play a significant role in the chemical industry. And many fine chemical reagents are mainly separated from coal tar, also coal tar could be used in the preparation of fuel oil by hydrogenation [2–6]. According to the reference data [7,8], the production of LTCT and MTCT were about 2 million tons in 2008, and increased to 8.61 million tons in 2012 in China. Thus, considering the huge production of coal tar, the development of separation method for the different kinds of compounds with relative high content from coal tar is imperative. Coal tar is a complex mixture that includes many kinds of heteroatom compounds which are different types phenol compounds [9,10], basic nitrogen compounds [11–14], neutral nitrogen compounds [7,15], neutral aliphatic and aromatic compounds [16– 19], etc. The concrete composition of coal tar varies with different coal types and coal pyrolysis technologies. In chemical industry, the phenol compounds mainly derived from coal tar and have extensive applications [20–22]. Commonly, the separation of phenol compounds from coal tar can be achieved by traditional methods, in which some chemical reactions were performed with strong alkaline and acidic aqueous solutions [23,24]. Similarly, coal tar is also the main source of N-compounds, such as indole, carbazole, pyridine and quinoline (see Fig. 1), in the chemical industry, which have been extensively applied in the productions of medicine, pesticides, spice, dyestuff, and plastics. Particularly, due to the chemical properties of indole, numerous chemical reactions can be achieved. Such as electrophilic aromatic substitution reaction, oxidation reaction, or applied in generating derivatives for further organic synthesis [25]. There are many non-catalytic processes that have been adopted for the separation of N-compounds from coal tar, such as alkali fusion (potassium hydroxide) [26], sulfuric acid washing, solvent extraction [1,27], ion-exchange resin separation [28,29], and liquid-liquid extraction by volatile carboxylic acid [30]. Though the above methods are widely accepted, there still exist some drawbacks or limits because some processes discharge a large amount of waste water, the equipment are corroded, or the selectivity is lower. Thus, for separation of the N-compounds, more efficient and environmental friendly methods, which have lower cost and higher selectivity than the common adopted methods are needed. Generally, liquid-liquid extraction has been adopted in many separation fields. But for the separation of fine chemicals from coal tar, some drawbacks limit the application of this method, since the organic solvents used are volatile, and the selectivity and efficiency are low. Compared to the common organic solvents, ionic liquids (ILs) are boomingly investigated and applied in variety fields [31]. Due to its nonvolatile, good thermosability, strong dissolving capacity, designability, selectivity, and environment friendly, the applications of ILs in green chemical processes were investigated by many researchers [20,22,32–37]. To our knowledge, few investigations were done for the separation of N-compounds form coal tar. Jiao et al. [38] used several imidazolium-based ILs as extraction
agents to separate indole from wash oil. Habaki et al. [39] discussed the separation for the coal tar absorption oil by an ionic liquid supported liquid membrane. And Su et al. [40] used two series phosphate-based alkylimidazolium ILs, dialkyl and dihydrogen phosphate alkylimidazolium ILs to separate nitrogen compounds from the coal tar diesel fraction by extraction. All of them showed good results. Moreover, many studies for denitrogenation from diesel and gasoline by different ILs indicated the feasibility of liquid-liquid extraction by ILs [41–45]. In this work, the imidazolium-based ILs with different anions, Cl , Br , BF 4 , HSO4 , and CH3 COO were applied to extract neutral N-compounds, indole and carbazole, from the model coal tar oil, which were immiscible with the model oil. The aim is to select one or more possible ILs as solvents with higher efficiency and lower cost for the separation of the neutral N-compounds, indole and carbazole, from coal tar. Extraction efficiency (EE) and distribution coefficient (D) were adopted to evaluate the extraction ability of the investigated ILs. Then, the main conditions that could affect the extraction process were systematically studied, such as the extraction temperature, mass ratio of the extraction agent to model oil, extraction time, and initial N-content. The optimized experimental conditions and extraction mechanism were explored. The extraction mechanism was confirmed to be the formation of a hydrogen bond between ILs and neutral N-compounds. Meanwhile, the influence of the regeneration and the recycling times of the used ILs were also studied. 2. Experimental 2.1. Materials and preparation of the model oil All chemicals used in this work were obtained commercially, which were analytical pure reagents. The purities were all refer to mass fraction which were reported by the suppliers. All of the ILs were dried by a rotary evaporator at 150 °C in vacuum conditions (Shanghai Shensheng Tech. Co., Ltd, R205D). The other reagents were all used without purification. And the relevant detailed information of the reagents is listed in Table 1. Since coal tar consists of various kinds of components, which can be broadly divided into acidic, alkaline, aromatic, and neutral compounds, respectively. Thus, indole, carbazole, pyridine, quinoline naphthalene, and acenaphthene were selected to prepare the model coal tar oil. And methylbenzene was selected as the solvent and used to prepare the solution. All the selected components were weighed accurately and then added into a beaker. Therefore, two model oils were prepared which the mass ratio were 1:1:1:10 for indole, pyridine, quinoline, and naphthalene (model oil I), and 1:1:10:5 for carbazole, pyridine, naphthalene, and acenaphthene (model oil II), respectively. For the second model coal tar oil, acetone was added to the model oil to avoid the phase splitting due to the dissolving capacity of acenaphthene and carbazole. The concentrations of the two model coal tar oils are listed in Table 2. 2.2. Equipment and extraction procedure
Fig. 1. Chemical structures of four N-compounds from coal tar.
The liquid-liquid extraction experiments were carried out by using a customized jacketed glass vessel, which had a volume of 50 ml and was connected to thermostatted water that could keep a constant temperature. A suitable amount of model oil and IL were mixed in the glass vessel, which was equipped with magnetic stirrer, and kept at a constant temperature within ±0.1 K (SYC-15Bs, produced by Nanjing Huchuan Electronic Equipment Co., Ltd.). During the experiment, the magnetic stirrer was set at 600 rpm at certain temperature for 30 min, which the temperature was set according to the different ILs, for instance, the temperature
29
L. Zhang et al. / Fuel 194 (2017) 27–35 Table 1 Suppliers and mass fractions of the chemical reagents. Component
CAS
Suppliers
Purity
Purification method
Methylbenzene Pyridine Quinoline Indole Carbazole Naphthalene Acenaphthene Acetone BmimCl BmimBr BmimBF4 BmimHSO4 BmimCH3COO
108-88-3 110-86-1 91-22-5 120-72-9 86-74-8 91-20-3 83-32-9 67-64-1 79917-90-1 85100-77-2 174501-65-6 262297-13-2 284049-75-8
Tianjin Kemiou Chemical Reagent Co., Ltd. Tianjin Fuyu Fine Chemical Co., Ltd. Tianjin Bodi Chemical Co., Ltd. Shanghai Zhanyun Chemical Co., Ltd. Shandong Xiya Chemical Industry Co Ltd. Chengdu Kelong Chemical Co., Ltd. Beijing Blingwei Technology Co., Ltd. Tianjin Kemiou Chemical Reagent Co., Ltd. Linzhoukeneng material Co., Ltd Linzhoukeneng material Co., Ltd Linzhoukeneng material Co., Ltd Linzhoukeneng material Co., Ltd Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences
P99.5% P99.5% P99.0% 99.0% 99.0% P99.0% 98.0% P99.5% 99.0% 99.0% 99.0% 98.0% 99.0%
None None None None None None None None Dried Dried Dried Dried Dried
sented the extraction performance of indole and carbazole by ILs. Both of the two equations are expressed as follows:
Table 2 Concentrations of two model coal tar oils. Components
Model oil I
Model oil II
Indole Pyridine Quinoline Naphthalene Carbazole Acenaphthene
58.19 g L1 39.43 g L1 64.59 g L1 192.37 g L1 – –
– 40.34 g L1 – 401.03 g L1 39.90 g L1 160.74 g L1
range was 40–100 °C to extract indole from the model oil by BmimCl. Then, the stirring was stopped and the mixture was settled for 1 h at the same temperature. During the settlement, any disturbance from the outside should be avoided until the phase equilibrium state was reached. After that, the two layers were separated carefully and the mass of both layers were all weighted accurately. The samples of the upper phase were drawn by syringes which were analyzed accurately by GC. The procedures were similar to Hansmeier’s work [41]. In addition, the back-extraction of the lower phase were adopted by water. 2.3. Analysis All samples of the upper phase were analyzed by a gas chromatography (Lunan GC SP-6890) which was equipped with a flame ionization detector (FID) and a capillary column (DB-WAX, 30 m 0.53 mm 1.00 lm, Agilent Technologies). The analysis conditions were described as follows: the temperature of the injector, 280 °C, the temperature of the FID detector, 270 °C and the detector rang for FID, 100. Considering the large boiling-point difference for different components, a temperature programming procedure was selected here. For the model oil I, the initial temperature of oven was set at 80 °C for 2 min, then the temperature increased at a rate of 30 °C min1 to reach 200 °C, and kept for 3 min; for the model oil II, the initial temperature of oven was set at 80 °C for 2 min, then the temperature increased at a rate of 30 °C min1 to reach 220 °C, and kept for 15 min. With the N2000 workstation developed by Zhejiang University, the concentration of all samples were determined, which was calibrated by four different known mixtures gravimetrically weighed by an analytical balance with a standard uncertainty of ±0.0001 g. For the purpose of lower deviation, each sample was analyzed by GC at least three times and the mean values were adopted. 2.4. Calculation of extraction efficiency and distribution coefficient The extraction efficiency (EE) were calculated by Eq. (1), and the distribution coefficient (D) were calculated by Eq. (2), which pre-
EENcompounds ¼ ½ðC 0 C f Þ=C 0 100
ð1Þ
DNcompounds ¼ C IL =C f
ð2Þ
where in Eqs. (1) and (2), C 0 is the concentration of the original model oil and C f is concentration of the model oil after extraction. In Eq. (2) C IL is the concentration of the model oil in IL phase. And the concentration of the lower layer can be obtained by the conservation of mass. 3. Results and discussion With different combination of the cations and anions, there are vast amount of ILs that can be developed theoretically and may have extensive applications in different fields. Several researchers have tried different types of ionic liquids for the separation or removal of N-compounds from coal tar or diesel fuels [38–40,42,43,45]. Thereinto, the performance of those ILs used in the extraction process for the neutral N-compounds, which were evaluated by one extraction step, are compared and summarized in Table 3. Although there have already some investigations by some researchers, the applications of the ILs are still limited by the high cost and some related performance, only little kinds of ILs have been applied in the industry, such as the imidazolium, pyri dinium cations with halide, BF 4 or PF6 anions. So it is important to select some appropriate kinds of ILs, which could have high efficiency and low cost, to extract neutral N-compounds from coal tar. 3.1. Selection of ILs For the studied model coal tar oils, it was found that the imidazole-based ILs had the ability to extract the neutral N compounds. The ILs with BF 4 or PF6 anions could be decomposed, which corrosive HF can be generated [46–48]. Thus, alternative anions, anions, such as Cl , Br and two acidic HSO 4 , CH3 COO were studied here. Meanwhile, ILs with BF4 was also checked for comparison. The ILs with same cation, 1-butyl-3-methylimidazolium, and different anions were investigated to extract neutral N-compounds from the two model coal tar oils. The structures of the five imidazole-based ILs are shown in Fig. 2, which are 1-butyl-3-methyl-imidazolium chloride (BmimCl), 1-butyl-3methyl-imidazolium bromide (BmimBr), 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4), 1-butyl-3-methylimidazolium disulfate (BmimHSO4), and 1-butyl-3-methylimidazolium acetate (BmimCH3COO), respectively. To select the suitable ILs, the experiments were performed with mass ratio of 1:1 for different ILs to the model oil in the glass vessel
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Table 3 Comparison of some ILs for the extraction of N-compounds. ILs
N-compounds
T/K
IL:oil
Extraction time/min
Extraction efficiency/%
Ref.
BmimBF4 BmimPF6 BmimH2PO4 BmimN(CN)2 C2mimCl (CH3CH2)3N(CH2)3SO3HHSO4
Indole Indole Coal tar diesel fraction Carbazole Indole Indole
303.15 303.15 313.15 298.15 333.15 298.15
1:1 1:1 1:5 1:1 1:1 1:5
60 60 30 20 >30 90
91.36 86.47 92.3 100 76 99.1
[38] [38] [40] [42] [43] [45]
Fig. 2. Chemical structures of selected ionic liquids: (1) 1-butyl-3-methyl-imidazolium chloride (BmimCl); (2) 1-butyl-3-methyl-imidazolium bromide (BmimBr); (3) 1-butyl-3-methyl-imidazolium tetrafluoroborate (BmimBF4); (4) 1-butyl-3methyl-imidazolium disulfate (BmimHSO4); (5) 1-butyl-3-methyl-imidazolium acetate (BmimCH3COO).
at 60 °C for 1 h with vigorous stirring. The calculated extraction efficiency and distribution coefficient for the N-compounds are given in Fig. 3. It was shown that, in Fig. 3, the different anions of the ILs presented different extraction effects, which the EE and D values for the neutral N-compounds followed the order of anions: BmimCl > BmimBr > BmimHSO4 > BmimBF4 > BmimCH3COO. Moreover, the EE by BmimCl and BmimBr were 97.50, 94.70 for indole and 95.01, 85.15 for carbazole, the D values were 38.98, 17.89 for indole and 19.10, 5.76 for carbazole. The EE and D values for pyridine and quinolone were obtained to make a comparison, which the EE values were 13.24, 8.25 for pyridine and 26.40, 11.45 for quinoline, the D values were 0.15, 0.09 for pyridine and
0.36, 0.13 for quinoline by BmimCl and BmimBr in the above conditions. From the above results, good selectivity of BmimCl for indole and carbazole could be found. Thus, BmimCl was more suitable to extract neutral N-compounds from the model oils. On the contrary, all the selected ILs presented little or none extraction ability for naphthalene and acenaphthene. In addition, BmimHSO4 had a relatively higher extraction ability than BmimCl for pyridine and quinoline, which the EE could reach 83.46 for pyridine and 59.99 for quinoline. But for indole and carbazole, the extraction ability of BmimHSO4 was smaller than that of BmimCl, which the EE were 89.24 for indole and 82.13 for carbazole. Hence, based on the overall extraction efficiency for N-compounds from coal tar, the optimized extraction process utilizing BminCl for the neutral N-compounds from the model coal tar oils were explored. Due to the high extraction efficiency and distribution coefficients of BmimCl for indole and carbazole, a promising way was presented to separate the neutral N-compounds from the model coal tar oils. Hence, the different conditions, such as the extraction temperature, mass ratio of IL to model oil, extraction time, and initial N-content, that may have an influence on the extraction process by BmimCl were optimized in detail in the following part. 3.2. Optimization of extraction conditions 3.2.1. Effect of extraction temperature The influence of extraction temperature was investigated in detail by the single factor experiments. Five temperature points were designated, which were 40 °C, 50 °C, 60 °C, 80 °C, 100 °C,
Fig. 3. The extraction efficiency and distribution coefficients of different N-compounds by five ionic liquids.
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respectively. The extraction was conducted with the mass ratio of 1:5 for 1 h. The EE and D values are presented in Fig. 4a, which indicated that the EE of indole increased slightly with the increasing of the temperature from 50 °C to 80 °C, then decreased over 80 °C. This could be caused by the solubility of indole in the methylbenzene increased when the temperature was larger than 80 °C. For carbazole, EE was insensitive to the temperature from 40 °C to 100 °C. And the D values of indole and carbazole presented the same trends as EE. For indole and carbazole, the maximum EE and D values were 89.64, 81.86 and 44.17, 21.58 at 80 °C, which only a little higher than those at 60 °C. Thus, in consideration of the industrial application, the optimum temperature was confirmed as 60 °C. And the extraction temperature was set at 60 °C for the following experiments.
3.2.2. Effect of mass ratio of IL to model oil The mass ratio of the IL to the model oil had a significantly influence on the extraction process. Therefore, the effect of mass ratio was investigated in detail and the mass ration of IL to model oil 1:10, 1:5, 1:2, 1:1, 2:1 (w:w) were selected. The extraction experiment was carried out at 60 °C for 1 h. The EE and D results indicated, as shown in Fig. 4b, that the EE values of indole and carbazole had the same trend, which increased with increasing the mass ratio, and EE increased fast from the mass ratio 1:10 to 1:2. Eventually, the EE values of indole and carbazole reached to 98.32 and 96.65, respectively at the mass ratio of 2:1. On the contrary, D for indole and carbazole had different trend with each other, which D for indole increased with increasing the mass ration and reached the maximum value 42.20 at the mass ratio of 1:2, but for carbazole, it decreased with the increase of the mass ratio from 1:10 to 2:1. Thus, considering the extraction effect and the cost of the ionic liquid, the mass ratio of 1:5 was selected in the following experiments.
3.2.3. Effect of extraction time The influence of extraction time was investigated in detail from 5 to 90 min, which 5, 10, 30, 60, 90 min were selected. All the extraction experiments were conducted at 60 °C with the mass ratio of 1:5. The results are presented in Fig. 4c, which the EE and D values for both indole and carbazole increased with increasing the extraction time. The EE and D values increased rapidly and reached to 90.10, 47.09 for indole and 83.14, 27.66 for carbazole, respectively, as the extraction time increased to 30 min. Then, the EE and D curves flattened as the extraction time increased to 90 min. According to the above experiment results, at least 10 min was needed for the extraction to reach the higher extraction efficiency. Therefore, 30 min was adopted to assure the extraction in this study.
3.2.4. Effect of initial neutral N-content Considering the different contents of neutral N-compounds in the different coal tar, several model oils with different initial content for the neutral N-compounds were prepared to explore the influence of the initial neutral N-content for the extraction by BmimCl, which the initial neutral N-content varied from 2% to 16%. Moreover, the extraction experiments were performed under the mass ratio of 1:5 (w/w) for ILs to model oil at 60 °C for 30 min. As shown in Fig. 4d, both EE and D had the same trend which decreased slightly with the increasing the initial neutral Ncontent. The extraction efficiency varied from 90.99% to 86.83% for indole and 83.43% to 76.47% for carbazole with the change of the N-content from 2% to 10%, which indicates the different initial contents of indole and carbazole in different model oils have less influence on the extraction process.
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3.3. Mechanism of the extraction process Based on the above experimental results, the interactions between the ILs and the neutral N-compounds accounted for the high extraction efficiency and selectivity of indole and carbazole. Many researchers have reported that the interactions existed between the ILs and aromatic hydrocarbons are p-p interactions by a so-called sandwich-like structure [32,41,49], the clathrate formed between the cations of the ILs and the enclosed solvent [50], or hydrogen bonds caused by the anions of the ILs [51,52], which may give an explanation for the high extraction ability of the investigated ILs. In addition, Wang et al. [47], regarded the extraction mechanism was not only liquid-liquid extraction, but also the acidity of the ILs. In this study, the same cation, 1-butyl-3-methyl-imidazolium, was adopted to reduce the other interaction influences. And different anions were adopted to investigate the extraction of neutral Ncompounds from the two model coal tar oils. The investigated ILs, BmimCl and BmimHSO4, had a higher extraction ability to extract indole and carbazole from the model oils, which could be explained by the intermolecular forces between the ILs and the neutral Ncompounds. Hetero-atoms, in indole and carbazole, had the possibility to form hydrogen bond with the cations or anions of different ILs. To verify the hypothesis, the chemical bonds formed between the ILs and the neutral N-compounds were simulated by using the DMol3 module incorporated the density functional theory (DFT) [53] which is available in Accelrys’ Materials studio. First, the geometry optimization of the neutral N-compounds, ILs and their complexus were done by the GGA/VWN-BP function with the DNP 4.4 basis set. To achieve rapid convergence, the smearing energy was set at 0.005 Ha. Meanwhile, the convergence criteria self-consistent field (SCF) tolerance was set at 1.0 ⁄ 106 Ha, and the convergence tolerance were set as follows: energy tolerance of 1.0 ⁄ 105, maximum force of 0.002 Ha/Å, maximum displacement of 0.005 Å. The dielectric constant of the solvent, methylbenzene, was 2.4. The parameter ‘‘Charge” were set as 1 or 1 for the cations and anions of different ionic liquids, respectively. As shown in Fig. 5, the bond lengths are 2.119 Å between BmimCl and indole (a), 2.126 Å between BmimCl and carbazole (b), respectively, which indicated that a hydrogen bond, NAH Cl, was formed between the anion and indole/carbazole. Meanwhile, in order to confirm the hydrogen bond between the anion of the IL and the neutral N-compounds, the electron densities are shown in Figs. 6 and 7, which reflected the formation of the hydrogen bond. The total densities, as shown in Fig. 6, indicate that the NAH Cl hydrogen bonds between indole/carbazole and anion are still evident, even if the isovalue is 0.2. Also, the similar result was obtained by the deformation density as shown in Fig. 7. In Fig. 7, a certain degree of H Cl bonding interactions is observed from the slice of the deformation density, which the received electron area encircling around Cl atom is expressed in red and the betatopic area encircling around H atom is expressed in blue for indole and carbazole with the anion. Similarly, a hydrogen bond was formed between indole/carbazole and BmimHSO4 with the bond lengths of 1.894 Å, 1.899 Å, respectively. And the hydrogen bonds of NAH O between indole/carbazole and anion are evident even if the isovalue is 0.15, which the corresponding bond lengths and electron density maps are both presented in the Supplementary Material (Fig. A. 1–3). On the contrary, pyridine and quinoline, which belongs to basic N-compounds, could not form the hydrogen bond with the anions due to the lack of hydrogen atom connected to the nitrogen atom. Therefore, based on the MS simulation results, the formation of the hydrogen bond between the anions and the hetero-atom presents the mechanism of the extraction process.
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Fig. 4. EE and D values vs. temperature (a), mass ratio of IL to model oil (b), extraction time (c), and initial N-content (d) for the extraction of indole (j) and carbazole (N).
Fig. 5. Bond lengths between BmimCl and indole (a), and between BmimCl and carbazole (b).
L. Zhang et al. / Fuel 194 (2017) 27–35
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Fig. 6. Total density maps of different isovalues for BmimCl with Indole (a, b, c) and Carbazole (d, e, f).
Fig. 7. Deformation charge density maps for BmimCl with Indole (a) and Carbazole (b).
3.4. Regeneration and recycling ionic liquid Considering the application of the extraction of indole and carbazole from coal tar oil by ILs in the industry, it is of importance to regenerate and reuse the ILs when the extraction process is applied. Eßer et al. reported that the neutral N-compounds could be separated from the extract by distillation [54]. In view of the high boiling temperatures of indole and carbazole, a backextraction process for indole and carbazole was adopted here. For the back-extraction process, water was selected as the backextraction agent. When the two phases were formed after the extraction, the upper was the oil phase and the lower was IL phase with the extracted neutral N-compounds. The two phases were separated by a separating funnel. Then, a certain amount of water was added into the IL phase with continuous stirring. After settling for a certain time, the neutral N-compounds precipitated and were
filtrated. The ILs was regenerated by a rotary evaporator at 150 °C in vacuum conditions, until the mass of the ILs was constant. The recycled BmimCl was reused to extract the neutral Ncompounds from the model oils. The extraction efficiencies of neutral N-compounds were not lowered even recycled 5 times, which is shown in Fig. 8. The IL could be easily regenerated and be sustainable recycled with good extraction performance. Meanwhile, the regenerated IL was checked by FT-IR, which is shown in Fig. 9. The spectra of the recycled IL were compared with the new IL, which the absorption peaks had no difference.
4. Conclusion In this work, an approach was provided for the separation of neutral N-compounds, indole and carbazole, from coal tar by ionic
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The extraction efficiency of BmimCl was not changed obviously and can be sustainable recycled with good extraction performance even it was recycled five times.
Acknowledgement The authors are grateful to the support of National Natural Science Foundation of China [Grant No. 21306106] and Shenzhen Supercomputer Center (DMol3 and Reflex modules of Materials Studio 7.0).
Appendix A. Supplementary material
Fig. 8. Extraction efficiency vs. recycle times of BmimCl for indole and carbazole model oils (temperature, 60 °C; mass ratio of IL to oil, 1:1; time, 1 h).
The bond lengths and electron density maps of the hydrogen bonds of NAH O between indole/carbazole and BmimHSO4 are both presented in the supplementary material. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.12.095.
References
Fig. 9. The FT-IR spectroscopy of the original IL and recycled IL.
liquids. The extraction effect of five imidazolium-based ionic liquids with different anions, BmimCl, BmimBr, BmimBF4, BmimHSO4, and BmimCH3COO, were explored. All of those five ILs had the extraction ability for the neutral N-compounds. The extraction efficiency and distribution coefficient were adopted to evaluate the extraction ability, which the extraction performance follows the order of anions: Cl > Br > HSO 4 > BF4 CH3 COO . And the high extraction ability and selectivity was confirmed for the neutral N-compounds by BmimCl from the model coal tar oil, which the distribution coefficient were 38.98 and 19.10 for indole and carbazole, 0.15, 0.36 for pyridine and quinoline, respectively. Furthermore, the conditions of the extraction were investigated in detail. The extraction efficiency could reach 98.32 and 96.65 for indole and carbazole at 60 °C with mass ratio of BmimCl to model coal tar oil, 2:1, and extraction time, 1 h. And the EE and D values decreased slightly with increasing of the initial N-content. The mechanism of the extraction of indole and carbazole from coal tar oil by ILs was confirmed to be the formation of the hydrogen bonds between ILs and neutral N-compounds, which were simulated by using the DMol3 module. Meanwhile, the regeneration and recycle of the BmimCl were investigated and water was selected as the back-extraction agent.
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